Modeling for Locating Pin Adjustment on Fixtures to Improve Positioning Accuracy of Parts

Many machining operations require fixtures for part positioning and clamping on CNC machine tools. To achieve required positioning accuracy of the part relative to the machine tool, the fixture usually provide locating pins which are in contact with some datum surfaces on the part. Traditionally, high precision is required for the manufacturing and assembly of the fixture for the part positioning accuracy. With the development of CNC techniques, the positioning error of the part can be compensated by the machine tool to lower the accuracy requirement for the fixture. However, six or more axes are required for the machine tool to compensate for all the position and orientation errors. This paper focuses on the usage of adjustable sliding pins as an alternative method for positioning error compensation for the machine tools that are lack of sufficient mobility or fixture compensation function. In particular, this paper proposes a generic mathematical model to calculate the amount of locating pin adjustment for compensating given positioning error of part. For this purpose, the kinematic equations of the part in contact with adjustable locating pins are derived, in which the pin positions are taken into account. Then the influence of the deviations in the pin positions on the part position and orientation is formulated. The model can be used to adjust the pins for positioning error compensation for all or some degrees of freedom of the part. A simulation study on a polyhedron part with three fixed and three adjustable pins for a drilling operation has been carried out to demonstrate the effectiveness of the proposed method.